That's the way it works. The trace tool draws a line at each edge between colors. A line has two edges so there it gets two traces.
JPGs have fuzzy edges, so the trace function does not produce clean lines. After tracing, use the Fit Curves to Vectors (Bezier Curves) to smooth the lines. Node editing can remove squiggles, too. Delete the nodes that make the squiggles. Or just trace the line. I often find that faster.
Each line that you traced will produce a very thin closed vector. If you want a single line, use the node editor to cut the closed vector at both ends and then delete one half. If the line was a closed figure to start with, just delete one of the lines.
Good judgement comes from experience.
Experience comes from bad judgement.